Hardware zero-cross event detection protection on ST7MC

[jon239955]

Posted on October 25, 2010 at 21:55

Hardware zero-cross event detection protection on ST7MC

[jon239955]

Posted on May 17, 2011 at 10:20

Gigi,

This is extremely helpful.  Thank you so much for sharing your experience with me.  After further experimentation, I found the same situation that you did: if I was VERY particular about my startup settings, I could make the motor start with Z protection enabled.  Once the motor started, I could run at many speeds reliably with the protection enabled.  Getting perfect startup settings with Z protection enabled is very tricky for my motor, however.  This matches your experience and your explanation makes complete sense to me.

I appreciate the note about the ST7MC and future designs as well.  Right now I am in a prototype phase trying to make the best of what I have.  I suspect I'll move to a newer micro if the prototype is successful.

Thank you again for the valuable feedback.

[Gigi]

Posted on May 17, 2011 at 10:20

Ciao,

as you write when the hardware zero-cross detection protection is enabled the zero crossing is validated only if a true edge is detected. So if a falling edge is expected the zero crossing is validated only if a low value is detected after one high value has been detected before.

If it is not enabled each low value is take as falling edge.

This mechanism assure that a true edge occurs and it should be ''theoretically'' the best but as you mention the start-up is ''in practice'' better without this setting.

The point is that is some condition, for instance during startup, the demagnetization phase can be to long to open correctly the BEMF detecting window. In this case you have no ''true'' zero crossing detected and the motor stops.

In this case I was able to run the motor disabling that feature and using the end of demagnetization as ''virtual'' zero crossing detection. The microcontroller in this way knows (at least) that a zero crossing occurs.

I hope that my experience on that issue answer to you question.

I want also underline that ST7MC is no more available for future designs but the problematic is the same for any 6-step control. In the STM8 FW library the ''True'' BEMF detecting has not be implemented for that reason.

Ciao

Gigi